Abs tract Aggregatibacter actinomycetemcomitans (Aa) and other bacteria attach to biotic and abiotic surfaces and produce exopolysaccharides, which are used to immobilize the bacterial cells on these colonized surfaces forming a biofilm matrix. The exopolysaccharide produced by Aa is a homopolymer of N-acetyl-D- glucosamine (GlcNAc) with b-1,6-linked units (abbreviated as PGA). During biofilm growth, to overcome starvation associated with overpopulation, cells must disperse from a surface to move to distal surfaces to colonize further. This movement of released cells of Aa is facilitated by the hydrolytic action of Dispersin B (DspB), an enzyme produced by Aa, which releases cells that are enmeshed in the biofilm matrix. The product of PGA hydrolysis, GlcNAc, is present around the mature biofilm and could become a nutrient for Aa and other neighboring bacteria. In a multispecies biofilm where many bacteria co-habitat, Aa gains nutrients by positioning itself near the commensal bacterium, Streptococcus gordonii (Sg). In this scenario, Aa can utilize the lactate produced by Sg, a preferred carbon source, for its growth. In order to maintain the presence of Sg in its vicinity, in reciprocation, Aa supplies the neighboring Sg with GlcNAc through the action of DspB on PGA. Since GlcNAc is a preferred carbon source for Sg, both bacteria can survive by entering into a nutritional co-operation wherein both have an advantage for persistent colonization in the niche. It should be noted that mutualistic or cooperative cross-feeding between bacteria is favored in biofilms (2). Our hypothesis is that 1) PGA-derived GlcNAc, not exogenous GlcNAc, is important for the survival and colonization of Aa in the biofilm and metabolic cross-feeding relationship with Sg; 2) DspB is required for the virulence of Aa. We will test these hypotheses with the following Specific Aims: Specific Aim 1: a) Demonstrate that DspB is essential for the survival of Aa in a biofilm mode of growth. b) Demonstrate that DspB is essential to maintain the cross-feeding relationship and cooperative nutrient exchange with Sg. Specific Aim 2: Characterize the role of DspB in the colonization and virulence of Aa in the oral cavity.